This invention is directed to phase analog servo systems and, more particularly, to following error measurement systems for phase analog servo systems.
Phase analog servo systems have found a variety of uses in various fields, particularly in the numerically controlled machine tool field. While a wide variety of phase analog servo systems have been developed and are in use, they all operate in a similar manner. In general, a position command signal is phase compared with a feedback signal. The phase difference causes the movement of a shaft. A resolver connected to the shaft moves the feedback signal in the direction of phase balance. The position command signal may be continuously changing or may only command a shaft position change at desired points. In the former case, the actual shaft position is usually different than the "commanded" shaft position at any particular point in time. In the latter case, the actual shaft position may achieve the commanded position sometime subsequent to the command; however, during movement there still exists a difference between the commanded position and the actual position. This position difference is referred to as following error and may be a leading or lagging difference depending upon the direction of movement.
It is pointed out here that knowledge of following error is of extreme importance to users of phase analog servo systems. More specifically, as briefly noted above, phase analog servo systems are used to control the automatic operation of machines. Usually such systems are utilized to control several independent machine axes. The systems control various axis operations in accordance with a "program". The program creates position commands which in turn control the operation of the phase analog servo systems in the manner herein described. Since a following error exists in phase analog servo systems, this error must be matched between all axes of machine motion. If it is not, the desired machine accuracy will not be achieved. Thus, following error information is necessary to correctly adjust servo systems.
In many phase analog servo systems the position command signal is derived from a series of command pulses. The command pulses modulate the output of an oscillator in a manner such that the output of the modulator changes from a fixed quiescent frequency. The direction of change (increase or decrease) determines the commanded shaft direction change; the number of command pulses determines the amount of the change; and the command pulse rate determines the rate of shaft position change i.e., the velocity of shaft movement. In one specific form, the modulated pulses are counted by a command counter and the resultant pulse count forms the command signal. Because the modulated pulses change in frequency, the output of the command counter changes in frequency, as well as in phase. The unmodulated oscillator signal is counted down and then phase modified in accordance with the position of the shaft to form a feedback signal. The thusly obtained command and feedback signals are phase compared and form a variable width error signal whose width is related to the phase difference between the two signals.
The herein described invention is primarily adapted to measure the following error of phase analog servo systems of the general nature described above; however, it can be used to measure the following error of other types of phase analog servo systems if suitable modifications, obvious to a person of ordinary skill in the art, are made to the illustrated and described structure to obtain the requisite signals.
The most commonly used prior art method of measuring following error uses a standard oscilloscope. A varying width error signal, resulting from comparing the command signal with the feedback signal is displayed, using a fixed time base. The time base is adjusted manually so that a predetermined number of divisions (such as 10) across the screen on the horizontal axis is equal to a fixed amount of error (usually 0.2 inches). Because calibration changes as a function of velocity (rate of command pulse application), the time base must be recalibrated each time a different following error is to be measured (i.e., each time there is a change in the rate at which command pulses are applied). Thus, measurements using this method are time consuming and, therefore, expensive to make. In addition, the accuracy of the resultant measurement data is totally dependent upon the testing individual's ability to adjust (calibrate) and interpret the displayed signal. Thus it is desirable to provide a digital following error measurement system which eliminates the "human" element and more rapidly performs the desired measurement.
Therefore, it is an object of this invention to provide a following error measurement system.
It is also an object of this invention to provide a measurement system suitable for measuring the following error of phase analog servo systems.
It is still another object of this invention to provide a following error measurement system suitable for measuring the following error of phase analog servo systems that provides a direct display of following error, and does not require recalibration each time a change occurs in the rate at which command pulses are applied to the phase analog servo system being tested.